Caldwell wasn't actually that surprising if you had been paying attention to what's going on in Detroit. The reason why Caldwell lasted even this long was due to typical Ford family meddling - which wouldn't necessarily be a problem if the Fords knew anything about football. This was all you need to know:
A few years back when former GM Martin Mayhew was fired and current GM Bob Quinn was hired Quinn was forced by Martha Ford to retain Caldwell. So not only is Caldwell not Quinn's guy, he was shoved down his throat. Sure, Caldwell has the highest winning percentage of any coach since the 1930s, but be honest - not a single one of those wins were meaningful in any way. Not to mention the Lions still haven't won a playoff game since the 1990s. The Lions are effectively wasting the best QB they likely have ever had to the point where a good number of Lions fans think Stafford is the problem despite just about anyone else nationally thinks Stafford is one of the best QBs in the league not named Brady or Rodgers. Add to this that, with Rodgers taken out of commission, this was the year the Lions had to do "it" - and they failed abysmally. And it wasn't just failing to win, but not ensuring the team was in position to succeed - like fielding a team not just with 10 players, but with 9 on occasion. Caldwell might be well known for getting along with his players and weekly prepration, but he is a terrible gameday coach and is regularly outcoached by his opponents.
I can vouch for that. Watch a replay of SB XLIV some time. Sean Payton coached circles around Caldwell. Nice guy and I get why players love him but he's in over his head on the sidelines.